What is a Denial of Service attack?

A Denial-of-Service (DoS) attack is one that causes the resources of a computer or server to be unavailable to its intended users. The term is most often used to refer to a malicious attempt to cripple an Internet site or web service from functioning optimally, but it can also refer to any attempt to limit the functionality of any computing resource. Common methods of attack include consuming so much of a computer/server's resources that it's effectively rendered inoperable or forcing it to reset. The term distributed denial-of-service (DDoS) is also often used to describe the above, and as its name suggests, it occurs when multiple systems attack the target system.